Continuous-flow machines, such as compressors or turbines, have rotationally fixed stator blades arranged in rings and rotor blades, which are firmly connected to the rotor, which can rotate, of the continuous-flow machine, in each case alternately in the flow channel. The radial gaps are formed between the radially outer tips of the rotor blades and the boundary surface, which is located radially on the outside, of the flow channel. Radial gaps are likewise formed between the tips of the stator blades and the inner boundary surface of the flow channel, which is formed by the outer surface of the rotor. Various methods are known for measurement of these radial gaps during operation.
U.S. Pat. No. 4,326,804 describes a method for radial gap measurement between the guide ring and the rotor blades of a turbine. A means which reflects light and reflects a measurement light beam, preferably laser light, is provided at each rotor-blade tip. The respectively reflected light beam is directed at a light spot position detector via a lens system. Its focus appears, as a function of the radial gap, at a position in the detector from which the radial gap is determined. In this case, one measurement is carried out per revolution for each rotor blade.
Furthermore, DE 27 30 508 discloses an optical method for determination of the distance between a stationary and a rotating component. A light source emits a conical light beam which is projected as a light spot of different size onto a light-intensive receiver as a function of the gap size, and this light spot is evaluated for distance measurement.
Furthermore, patent specification DE 196 01 225 C1 discloses an apparatus for radial-gap monitoring for a turbine, in which a measurement reference point for reflection of light is provided on a turbine blade, with the light being directed at the measurement reference point from a glass-fiber probe which is passed through the turbine casing. During operation of the turbine, the currently detected intensity differences between the transmitted light and received light are compared with the intensity differences determined in a reference measurement, and the size of the radial gap is calculated from the discrepancy in the intensity difference between real measurement and the reference.
Furthermore, EP 492 381 A2 discloses a method for tip clearance measurement on turbine blades using an optical transmitter and receiver, with the receiver receiving the light that has been reflected from the turbine blades and, in this case, evaluating the time reflection-intensity profile.
This method is based on a transmitter and a receiver in the form of a sensor being placed in the stationary system, that is to say in the outer boundary wall or in the casing, in order to use optical effects to identify the rotating component, which is thus moving past the receiver or the sensor tip, and to determine the distance to it at this instant.
In general, these methods are characterized in that the receivers or sensors that are used cannot be miniaturized below a specific limit and thus have a mass which cannot be ignored. Furthermore, some of the methods require complex feed and transmission electronics.
These sensors cannot be mounted at the tip of a free-standing stator blade in a continuous-flow machine since a sensor such as this would have a negative influence on the natural oscillation behavior of the stator blades. These stator blades could be caused to oscillate during operation, thus reducing the life of the blades.
It is often impossible to arrange sensors in the rotating system, or this requires an unjustified high degree of complexity in order to supply the generally complex electronics. If sensors, or in particular receivers, are provided in the rotating system, a costly telemetry installation, which is susceptible to defects, may be required in order to pass information out of the rotating system, and this increases the general complexity.